In general, a signal transmitted from a wireless transmitter reaches a wireless receiver as direct wave. In addition to or in place of the direct wave, one or more reflected waves reflected from buildings, obstacles, or the earth also reach the wireless receiver. The direct wave and/or the reflected waves constitute a series of arriving waves standing on the time axis, which then define a multi-path power delay profile. By accurately distinguishing the amplitudes and the timings of these paths, the original signal transmitted from a wireless transmitter can be restored to a satisfactory degree in wireless telecommunications.
Path timing in the power delay profile is detected using a technique called “path search”. In conventional path search, the threshold based on noise level is set to a certain level, and a path is detected based on whether the power level of the path exceeds the threshold based on the noise level. An example of the conventional path search technique is disclosed in JP 2001-217747A.
However, some of the path candidates appearing in the power delay profile arise from the sidelobe component of the path, differing from the true path timing. For example, when a path with relatively large power passes through a roll-off filter, the sidelobe component of this path is produced, and this sidelobe component may appear in the power delay profile. Such a sidelobe component may be neglected in conventional use or products; however, in future application to high-performance products, the sidelobe component may degrade the accuracy of path search. It is difficult to remove such an unnecessary path component by adjusting the threshold based on the noise level because if the threshold based on the noise level is set too low, many sidelobe components are picked up as true path components, while if the threshold based on the noise level is set too high, not only unnecessary path components, but also true path components are removed. In such a case, sidelobe components with a power level equivalent to that of true path components exist in the power delay profile.
Meanwhile, in multiple-input multiple-output (MIMO) systems designed to achieve high communication capacity, multiple transmission antennas and multiple receiving antennas are employed in a transmitter/receiver. Accordingly, multipath interference occurs not only in a propagation path from a transmission antenna 206-p to a certain receiving antenna 208-x, but also in a path from another transmission antenna 206-q to the receiving antenna 208x, as illustrated in FIG. 1 and FIG. 2. In a MIMO system, as the number of antennas increases, the path candidates also increase and a highly accurate path search is required. At the same time, degradation of path search accuracy is a concern.